The invention relates to processes for bag moulding thin articles made of a composite material consisting of layers of reinforcing fibres embedded in a matrix of resin to be polymerised, and more particularly to a moulding process using a shaper to fashion the corners of such articles.
Bag or bladder moulding is familiar to the skilled addressee and is used to mould thin-walled articles made of a composite polymerisable fibre resin material. This process uses an open mould having a moulding surface corresponding to the shape of the required article. Layers of fibres impregnated with polymerisable resin are placed on the moulding surface, the fibre layers are covered by fabrics which may or may not be contiguous and by a bladder or bag which is hermetically sealed off from the mould, an isostatic air pressure is applied to the bladder and the whole is treated by a polymerisation cycle which is usually performed hot.
The article may comprise projecting corners having a rounded vertex. To form these roundings correctly a shaper is so placed between the article and the bladder as to be in contact with the concave surface of the rounding. A shaper is a solid member which usually takes the form of a rod and which comprises a shaping surface engaged with the concave surface of the article in the hollow of and adjacent to the required rounding, the shaping surface having a shape matching the concave surface of the article--i.e., the shaping surface is convex and has a rounded vertex prolonged on either side by flanks, the shaper also comprising a surface which experiences the air pressure and which is remote from the shaping surface. The air pressure causes the shaper to urge the resin-impregnated fibre layers against the moulding surface and thus facilitates the proper shaping of the rounding in the corner of the article.
The disadvantage of the process is that the shaper edges mark the article and produce thereon abrupt changes in thickness which reduce the strength of the article. This is a particularly serious disadvantage in aeronautics since the articles are severely stressed and often have low-tolerance profiles.
This problem is solved by the use of shapers which are made of flexible elastomer and which have tapering bevel edges. The thickness inequalities are greatly reduced but not eliminated. Also, since high-strength resins require a high polymerisation temperature, shapers of this kind are soon damaged by the heat and require frequent renewal. Finally, the shapers are difficult to position in the corner of the article because of the large contact areas involved and because of pre-compression swelling of the fibres or layers of fabric.